1. Field of the Invention
The present invention relates to an image processing technique to reduce color blur in a color image produced by an image capturing system of an image pickup apparatus such as a video camera and a digital still camera.
2. Description of the Related Art
A color image produced by the above-mentioned image capturing system sometimes includes, around its high luminance portion, originally nonexistent color as color blur generated due to chromatic aberration of an imaging optical system (image taking optical system). The color blur is likely to occur at wavelengths away from a central wavelength of the imaging optical system. In a visible-light color image, a blue artifact, a red artifact or a purple artifact (purple fringe) that is a blend of the red and blue artifacts is generated as a blurred color.
A combination of plural lenses having mutually different dispersions can optically suppress the chromatic aberration of the imaging optical system to some extent. However, increase of resolution of an image sensor and reduction of size of the imaging optical system make it difficult to sufficiently suppress the chromatic aberration by optical approach. Thus, reduction of the color blur by image processing is desired.
The chromatic aberration is a phenomenon in which, for example, in an image whose green (G) plane corresponding to a central wavelength of a visible wavelength range is in focus, a red (R) plane and a blue (B) plane corresponding to edge side wavelengths thereof are out of focus. As an image processing method for correcting such chromatic aberration, Japanese Patent Laid-Open Nos. 2008-147980 and 2008-147981 disclose a method specifying a color blur correction area based on distance from a luminance saturation area or on intensity gradient (luminance gradient).
However, the method specifying the color blur correction area based only on the distance from the luminance saturation area regards whole pixels within a predetermined distance area near the saturated pixel as color blur pixels and corrects them. This method may provide an undesirable correction result in a case where the distance area is improperly set or depending on property of an object.
Moreover, the method specifying the color blur correction area based only on the intensity gradient regards whole pixels satisfying a predetermined intensity gradient condition as the color blur pixels and corrects them. This method may perform improper correction on object having the color distribution that distinction is impossible only in the intensity gradient condition and color blur not satisfying the intensity gradient condition locally due to noise.